Field of the Invention
The present invention relates to a surface emitting laser and an image forming apparatus using the surface emitting laser.
Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is utilized as a light source of a scanning optical device in an electrophotographic image forming apparatus. In this kind of scanning optical device, outgoing light from a light source is shaped by a diaphragm. The outgoing light is irradiated to a photoreceptor which is a scanned surface by an optical deflector (for example, a polygon mirror), to form a latent image.
When the surface emitting laser is utilized as the light source of the image forming apparatus, the surface emitting laser is desirably oscillated in only a basic transverse mode from the viewpoint of the monomodality and stability of the latent image in the photoreceptor. Japanese Patent Application Laid-Open No. 2001-284722 discusses a technique of oscillating a surface emitting laser in a single transverse mode by providing a stepped structure on the light outgoing surface of the surface emitting laser. The stepped structure is constituted so that the reflectance of the center portion of a light emitting region is higher than that of the peripheral portion. Because light intensity is distributed in the center portion in the fundamental transverse mode compared with a high order transverse mode, light in the fundamental transverse mode can be selectively oscillated by providing the stepped structure.
The stepped structure of Japanese Patent Application Laid-Open No. 2001-284722 is characterized that the reflectance in the center portion of the light emitting region is different from that in the peripheral portion. When the stepped structure is provided, the transmittance of light in the center portion of the light emitting region is different from that in the peripheral portion thereof in addition to the reflectance. A light path length difference in the center portion of the light emitting region is different from that in the peripheral portion thereof.
In the stepped structure in which the reflectance of the center portion is higher than that of the peripheral portion, the intensity of laser light transmitted from the peripheral portion is higher than that in the case where the stepped structure does not exist. The phase of light transmitted through the peripheral portion is different from the phase of light transmitted through the center portion. As a result, a near field light intensity distribution approaches to an annular form.
FIG. 9 illustrates a far field light intensity distribution corresponding to the near field light intensity distribution. Compared with a far field light intensity distribution 920 when the stepped structure is not provided, in a far field light intensity distribution 910 when the stepped structure is provided, the half value width of the far field light intensity distribution at the center of a divergence angle (example: −5 degrees to +5 degrees) is thinned. When the surface emitting laser is used as the light source of the image forming apparatus, accuracy required for the angle of the surface emitting laser attached to a scanning optical system of the image forming apparatus (axial alignment of an outgoing surface normal line of the light source and an optical axis of an optical system) is severer. Therefore, it is not desirable that the surface emitting laser is used.